The genophage bioweapon effectively sterilized the krogan by ensuring only one in a thousand pregnancies would come to term. Initially transmitted via a retrovirus deployed on Tuchanka, the genophage "ate" key genetic sequences, altering every cell in every krogan and creating a hereditary genetic flaw.
However, organic life thrives on adapting to adversity. Years after its deployment, salarian observers discovered that "garbage DNA" was cluttering the attachment sites originally used by the genophage, limiting its effectiveness and causing krogan birth rates to rise.
One krogan clan, Nakmor, showed persistent genophage adaptation and tried to commission studies into improving it. Attempts to cure the genophage are normally political or corporate suicide due to pressure from the Citadel Council, but the Andromeda Initiative operated outside those boundaries. After several trial runs, a gene therapy was developed to improve the Nakmor mutation and leverage the natural adaptability of krogan physiology. Initial signs are promising.